The object of this study is to understand whether phosphoprotein phosphatase participates in the regulation of protein phosphorylation. The phosphoprotein phosphatase is distributed in both soluble and particulate fractions of pineal glands. The properties of particulate bound and soluble phosphoprotein phosphatase are different. Fractionation of the cytosol indicated the existence of two forms of enzymes differing in molecular weight. A thermostable factor which activates the soluble enzyme was demonstrated in pineal glands and furthermore, it was shown to be a protein in nature. Whether the two different forms of phosphoprotein phosphatase and the thermostable activator of the enzyme mediate the regulation of phosphoprotein phosphatase remains to be further investigated. BIBLIOGRAPHIC REFERENCE: Yang, H. -Y. T., Costa, E., Majane, E.A. and Hong, J.S.: Phosphoprotein phosphatase of pineal gland: Some properties of the enzyme and the identification of an endogenous activator. J. Neurochem. 28: 1075-1080, 1977.